Why align-items-center doesn't work? [duplicate]

This question already has an answer here:

• Vertical Align Center in Bootstrap 4 6 answers

justify-content-center works properly but align-items-center not

HTML code:

<div class="container h-100">
   <!--Another content-->
  <div class="footer bg-dark ">
        <div class="row justify-content-center align-items-center">
        <span >&copy 2014 Keepsake. All rights reserved. Theme by 
         elemis</span>
    </div>
    </div>
</div>

Css:

div.footer{
width: 100%;
height: 150px;
span{
    color:white;
}
}